Magnetic tunnel junction having coherent tunneling structure

ABSTRACT

A magnetic tunnel junction includes an amorphous ferromagnetic reference layer having a first reference layer side and an opposing second reference layer side. The first reference layer side has a greater concentration of boron than the second reference layer side. A magnesium oxide tunnel barrier layer is disposed on the second side of the amorphous ferromagnetic reference layer. The magnesium oxide tunnel barrier layer has a crystal structure. An amorphous ferromagnetic free layer is disposed on the magnesium oxide tunnel barrier layer.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/501,535, filed Jul. 13, 2009. The entire disclosure of thisapplication is incorporated herein by reference.

BACKGROUND

A basic component of a magnetic tunnel junction (MTJ) is a sandwich oftwo thin ferromagnetic layers separated by a very thin insulating layerthrough which electrons can tunnel. The tunneling resistance is oftenlower when the magnetic moments of the ferromagnetic layers are paralleland higher when the magnetic moments of the two ferromagnetic layers areanti-parallel.

The change in conductance for these two magnetic states can be describedas a magneto-resistance. The tunneling magneto-resistance (TMR) of theMTJ can be defined as (R_(AP)-R_(P))/R_(P) where R_(P) and R_(AP) arethe resistance of the MTJ for parallel and anti-parallel alignment ofthe ferromagnetic layers, respectively. MTJ devices have been proposedas memory cells for nonvolatile solid state memory and as externalmagnetic field sensors, such as TMR read sensors for heads for magneticrecording systems. For a memory cell application, one of theferromagnetic layers in the MTJ is the reference layer and has itsmagnetic moment fixed or pinned via a synthetic ferromagnetic (SAF)layer and an anti-ferromagnetic (AFM) layer, so that its magnetic momentis unaffected by the presence of the magnetic fields applied to thedevice during its operation. The other ferromagnetic layer in thesandwich is the free layer, whose moment responds to an externalmagnetic field applied during operation of the device. In the quiescentstate, in the absence of any applied magnetic field within the memorycell, the free layer magnetic moment is designed to be either parallel(P) or anti-parallel (AP) to the magnetic moment of the referenceferromagnetic layer. For a TMR field sensor for read head applications,the reference ferromagnetic layer has its magnetic moment fixed orpinned via a synthetic ferromagnetic (SAF) layer and ananti-ferromagnetic (AFM) layer so as to be generally perpendicular tothe magnetic moment of the free or sensing ferromagnetic layer in theabsence of an external magnetic field.

For applications of magnetic tunnel junctions for either magneticrecording heads or for non-volatile magnetic memory storage cells, highTMR values are desired for improving the performance of these devices.

BRIEF SUMMARY

The present disclosure relates to magnetic tunnel junctions having acoherent tunneling structure. In particular, the present disclosurerelates to magnetic tunnel junctions that have coherent interfacesbetween the magnetic layers (free layer and reference layer) and thetunnel barrier layer and methods of forming the same.

In one particular embodiment, a magnetic tunnel junction includes anamorphous ferromagnetic reference layer having a first reference layerside and an opposing second reference layer side. The first referencelayer side has a greater concentration of boron than the secondreference layer side. A magnesium oxide tunnel barrier layer isdeposited on the second side of the amorphous ferromagnetic referencelayer. The magnesium oxide tunnel barrier layer has a crystal structure.An amorphous ferromagnetic free layer is then deposited on the magnesiumoxide tunnel barrier layer.

In another embodiment, a method includes the steps of depositing anamorphous ferromagnetic reference layer having a first reference layerside and an opposing second reference layer side. The first referenceside has a greater concentration of boron than the second referenceside. Then the method includes depositing a magnesium oxide tunnelbarrier layer disposed on the second side of the amorphous ferromagneticreference layer. The magnesium oxide tunnel barrier layer has a crystalstructure. Then the method includes depositing an amorphousferromagnetic free layer disposed on the magnesium oxide tunnel barrierlayer, forming a magnetic tunnel junction.

These and various other features and advantages will be apparent from areading of the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure may be more completely understood in consideration of thefollowing detailed description of various embodiments of the disclosurein connection with the accompanying drawings, in which:

FIG. 1 is a cross-sectional schematic diagram of an illustrativemagnetic tunnel junction;

FIG. 2 is a cross-sectional schematic diagram of another illustrativemagnetic tunnel junction;

FIG. 3A is a schematic diagram of a coherent interface of twocrystalline materials;

FIG. 3B is a schematic diagram of a semi-coherent interface of twocrystalline materials;

FIG. 3C is a schematic diagram of a incoherent interface of twocrystalline materials;

FIG. 4A is a schematic diagram of a illustrative magnetic tunneljunction prior to annealing;

FIG. 4B is a schematic diagram of an illustrative magnetic tunneljunction during annealing;

FIG. 4C is a graph of boron content as a function of position within theillustrative magnetic tunnel junction of FIG. 4A and FIG. 4B;

FIG. 5 is a flow diagram of an illustrative method of forming a magnetictunnel junction having coherent interfaces; and

FIG. 6A-6C are schematic diagrams for forming a magnetic tunnel junctionaccording to method described in FIG. 5.

The figures are not necessarily to scale. Like numbers used in thefigures refer to like components. However, it will be understood thatthe use of a number to refer to a component in a given figure is notintended to limit the component in another figure labeled with the samenumber.

DETAILED DESCRIPTION

In the following description, reference is made to the accompanying setof drawings that form a part hereof and in which are shown by way ofillustration several specific embodiments. It is to be understood thatother embodiments are contemplated and may be made without departingfrom the scope or spirit of the present disclosure. The followingdetailed description, therefore, is not to be taken in a limiting sense.The definitions provided herein are to facilitate understanding ofcertain terms used frequently herein and are not meant to limit thescope of the present disclosure.

Unless otherwise indicated, all numbers expressing feature sizes,amounts, and physical properties used in the specification and claimsare to be understood as being modified in all instances by the term“about.” Accordingly, unless indicated to the contrary, the numericalparameters set forth in the foregoing specification and attached claimsare approximations that can vary depending upon the desired propertiessought to be obtained by those skilled in the art utilizing theteachings disclosed herein.

The recitation of numerical ranges by endpoints includes all numberssubsumed within that range (e.g. 1 to 5 includes 1, 1.5, 2, 2.75, 3,3.80, 4, and 5) and any range within that range.

As used in this specification and the appended claims, the singularforms “a”, “an”, and “the” encompass embodiments having pluralreferents, unless the content clearly dictates otherwise. As used inthis specification and the appended claims, the term “or” is generallyemployed in its sense including “and/or” unless the content clearlydictates otherwise.

The present disclosure relates to magnetic tunnel junctions having acoherent tunneling structure. In particular, the present disclosurerelates to magnetic tunnel junctions that have coherent interfacesbetween magnetic layers (free layer and reference layer) and the tunnelbarrier layer and methods of forming the same. Reducing boronconcentration in the reference magnetic layer and free magnetic layer atthe tunnel barrier interfaces allows the formation of coherentinterfaces between these layers more easily. Coherent interfaces providesuperior tunneling magnetoresistance (TMR) and lower resistance areaproduct (RA) in the magnetic tunnel junction (MTJ) and also enable lowerannealing temperatures, or better interface quality at the sameannealing temperatures. The structures and methods described hereinensure crystallization (during the annealing process) of the amorphousfree and reference magnetic layers initiate from the tunnel barriercrystal structure and utilize the tunnel barrier crystal structure as atemplate for crystal structure growth outward from the tunnel barrier.While the present disclosure is not so limited, an appreciation ofvarious aspects of the disclosure will be gained through a discussion ofthe examples provided below.

For clarity, additional layers such as a capping layer, seed layer, andadditional pinning or reference layers are not illustrated in thefigures. It is understood that one or more of these additional layerswould be present.

FIG. 1 is a cross-sectional schematic diagram of an illustrativemagnetic tunnel junction 10.

As stated above, a cap layer, or additional reference layers (e.g., SAFand/or SAF) are not illustrated in these figures. The magnetic tunneljunction 10 includes an amorphous ferromagnetic reference layer 14having a first reference layer side 13 and an opposing second referencelayer side 15. The amorphous ferromagnetic reference layer 14 can bedeposited on a substrate 12 such as, for example AlTiC. The firstreference side 13 has a greater concentration of boron (B) than thesecond reference side 15. A magnesium oxide tunnel barrier layer 16 isdisposed on the second side 15 of the amorphous ferromagnetic referencelayer 14. The magnesium oxide tunnel barrier layer 16 has a crystalstructure such as a (001) crystal structure, for example. An amorphousor nano-crystal ferromagnetic free layer 18 is disposed on the magnesiumoxide tunnel barrier layer 16. The amorphous ferromagnetic free layer 18has a first free layer side 17 and an opposing second free layer side19. In some embodiments, the second free layer side 19 has a greaterconcentration of boron than the first free layer side 17. The first freelayer side 17 is in contact with the magnesium oxide tunnel barrierlayer 16.

The amorphous ferromagnetic reference layer 14 can be formed of anyuseful ferromagnetic material such as, for example, alloys of Co, Fe,and/or Ni, and the like, with elements like B, Si, Ta, Ti, Zr, Nb, andthe like, and Heusler alloys (e.g., Co₂MnSi, Co₂MnGe). In manyembodiments the amorphous or nano-crystal ferromagnetic reference layer14 comprises a CoFeB material. The amorphous ferromagnetic free layer 18can be formed of any useful ferromagnetic material such as, for example,alloys and materials including Co, Fe, and/or Ni and optionally withother elements such as Si, Ta, Ti, Zr, Nb, and the like, and Heusleralloys (e.g., Co₂MnSi, Co₂MnGe), for example. In many embodiments theamorphous ferromagnetic free layer 18 comprises a CoFeB material.

Tailoring the interfaces between the magnesium oxide tunnel barrierlayer 16 and the amorphous or nano-crystal ferromagnetic free layer 18and the amorphous ferromagnetic reference layer 14 has been found toimprove the tunneling magnetoresistance (TMR) and reduce the resistancearea product (RA) in the magnetic tunnel junction (MTJ) and also enableslower annealing temperatures. Forming a coherent interface (see FIG. 3A)between the magnesium oxide tunnel barrier layer 16 and the amorphousferromagnetic free layer 18 and the amorphous ferromagnetic referencelayer 14 enables higher tunneling magnetoresistance (TMR), reducedresistance area product (RA) and also enables lower annealingtemperatures. The coherent interfaces can be formed by initiatingcrystal structure formation of the amorphous ferromagnetic layers 14, 18at the interface with the magnesium oxide tunnel barrier layer 16 andutilizing the magnesium oxide tunnel barrier layer 16 crystal structure(such as the (001) crystal structure) as a template for crystalstructure formation.

In many embodiments, the amorphous ferromagnetic layers 14, 18 aredeposited such that a boron concentration in the amorphous ferromagneticlayers 14, 18 is a gradient that increases as it moves away from themagnesium oxide tunnel barrier layer 16 in a thickness direction of theamorphous ferromagnetic layers 14, 18. In many embodiments, theamorphous ferromagnetic reference layer 14 has a boron concentrationgradient extending between the first reference layer side 13 and theopposing second reference layer side 15. In some embodiments, theamorphous ferromagnetic free layer 18 has a boron concentration gradientextending between the first free layer side 19 and the opposing secondfree layer side 17. In many embodiments, the boron concentration in theamorphous ferromagnetic layers 14 and/or 18 can range from 0 to 15%atomic at the side near the magnesium oxide tunnel barrier layer 16 andfrom 10 to 25% atomic at the side further from the magnesium oxidetunnel barrier layer 16.

FIG. 2 is a cross-sectional schematic diagram of another illustrativemagnetic tunnel junction 20. The magnetic tunnel junction 20 includes amagnesium oxide tunnel barrier layer 16 separating an amorphousferromagnetic reference layer 14 from an amorphous ferromagnetic freelayer 18. The amorphous ferromagnetic reference layer 14 can bedeposited on a substrate 12 such as, for example AlTiC.

In these embodiments, the amorphous or nano-crystal ferromagnetic freelayer 18 and the amorphous ferromagnetic reference layer 14 can beformed of two or more sub-layers. These sub-layers can have the same ordifferent compositions. For example, the amorphous ferromagneticreference layer 14 is formed of a first reference sub-layer 14A having afirst boron concentration and a second reference sub-layer 14B having asecond boron concentration, and the first boron concentration is greaterthan the second boron concentration. In some embodiments the firstsub-layer 14A has a greater cobalt concentration than the secondsub-layer 14B. In some embodiments the second sub-layer 14B has agreater iron concentration than the first sub-layer 14A.

In some embodiments, the amorphous or nano-crystal ferromagnetic freelayer 18 is formed of a first free sub-layer 18A having a first boronconcentration and a second sub-layer 18B having a second boronconcentration, and the second boron concentration is greater than thefirst boron concentration. In other embodiments, the amorphousferromagnetic free layer 18 is formed of a first free sub-layer 18Ahaving a CoFeB material and a second sub-layer 18B having a NiFeM (whereM is any useful third alloying element enabling the amorphous nature ofthe alloy) amorphous magnetic material.

The ferromagnetic reference layer 14 determines the crystal structure ofthe magnesium oxide tunnel barrier layer 16 deposited on it. A magnesiumoxide tunnel barrier layer 16 having a desired crystal structure (001)develops on an amorphous surface and not on a crystalline surface. Thus,the ferromagnetic reference layer 14 should be amorphous when themagnesium oxide tunnel barrier layer 16 is deposited on it. In someembodiments, an amorphous magnetic or non-magnetic layer can bedeposited on the first reference sub-layer 14A to ensure the amorphousstructure formation of the second reference sub-layer 14B. In someembodiments, the first reference sub-layer 14A and/or the secondreference sub-layer 14B can be deposited at a low temperature such as,−50 degrees centigrade or less, to ensure the amorphous structureformation of the second reference sub-layer 14B.

FIG. 3A is a schematic diagram of a coherent interface of twocrystalline materials. FIG. 3B is a schematic diagram of a semi-coherentinterface of two crystalline materials. FIG. 3C is a schematic diagramof an incoherent interface of two crystalline materials. FIG. 3Aillustrates some strain as a result of the coherent interface betweenthe first crystal structure {acute over (α)} and the second crystalstructure β, having a different lattice constant, this is described as acoherent interface. FIG. 3B illustrates dislocations where the firstcrystal structure {acute over (α)} does not line up with the secondcrystal structure β, this is described as a semi-coherent interface.Dislocations are formed to relieve the strain between these two crystalstructures when their lattice constant mismatch is relatively largerthan those in FIG. 3A. FIG. 3C illustrates a majority of dislocationswhere the first crystal structure {acute over (α)} does not line up withthe second crystal structure β, this is described as an incoherentinterface. This is the case when the two crystal structure has a verylarge lattice mismatch. Coherent interfaces between the tunnel barrierand the ferromagnetic free and reference layers of a magnetic tunneljunction have been found to dramatically increase the tunnelingmagnetoresistance (TMR) in the magnetic tunnel junction (MTJ).

FIG. 4A is a schematic diagram of an illustrative magnetic tunneljunction prior to annealing. FIG. 4B is a schematic diagram of anillustrative magnetic tunnel junction during annealing. FIG. 4C is agraph of boron content as a function of position within the illustrativemagnetic tunnel junction of FIG. 4A and FIG. 4B.

As described above, the magnetic tunnel junction includes a magnesiumoxide tunnel barrier layer 16 separating an amorphous ferromagneticreference layer 14 from an amorphous ferromagnetic free layer 18. Themagnesium oxide tunnel barrier layer 16 possesses a (001) crystalstructure. As illustrated in FIG. 4C, boron content or concentrationdecreases (which can be in part due to boron diffusion into the MgObarrier) at the interfaces between the magnesium oxide tunnel barrierlayer 16 and the amorphous ferromagnetic reference layer 14 and theamorphous ferromagnetic free layer 18. Due to the lower boronconcentration at these interfaces, crystallization initiates at theseinterfaces at a lower temperature and grows outward from theseinterfaces as illustrated in FIG. 4B. In many embodiments,crystallization initiates at these interfaces at a temperature of 325degrees centigrade or less, or at a temperature of 300 degreescentigrade or less, or at a temperature of 275 degrees centigrade orless.

Care is taken to reduce or prevent crystallization initiation within theamorphous ferromagnetic reference layer 14 or from the Ru (within SAF)and reference interface or the amorphous ferromagnetic free layer 18 orfrom the free layer 18 and capping layer interface except at theinterface with the magnesium oxide tunnel barrier layer 16. Thus,depositing a magnetic or non-magnetic amorphous layer adjacent to theamorphous ferromagnetic reference layer 14 and the amorphousferromagnetic free layer 18 can assist in preventing or suppressingcrystallization initiation within the amorphous ferromagnetic referencelayer 14 or the amorphous ferromagnetic free layer 18 except at theinterface with the magnesium oxide tunnel barrier layer 16. In someembodiments the entire ferromagnetic reference layer 14 and theferromagnetic free layer 18 forms a crystal structure. In otherembodiments, only a portion of the ferromagnetic reference layer 14 andthe ferromagnetic free layer 18 forms a crystal structure.

FIG. 5 is a flow diagram of an illustrative method of forming a magnetictunnel junction having coherent interfaces. FIG. 6A-6C are schematicdiagrams for forming a magnetic tunnel junction according to methoddescribed in FIG. 5. The method 100 includes depositing an amorphousferromagnetic reference layer having a first reference layer side and anopposing second reference layer side where the first reference sidehaving a greater concentration of boron than the second reference sideat block 101. FIG. 6A illustrates the amorphous ferromagnetic referencelayer 14 deposited on a substrate 12. The reference layer 14 can includea number of additional layers between the amorphous ferromagneticreference layer 14 and the substrate 12, as further described below.

Then the method includes depositing a magnesium oxide tunnel barrierlayer disposed on the second side of the amorphous ferromagneticreference layer where the magnesium oxide tunnel barrier layer has a(001) crystal structure, at block 102. FIG. 6B illustrates the magnesiumoxide tunnel barrier layer 16 on the amorphous ferromagnetic referencelayer 14. An amorphous or nano-crystal free layer is then deposited onthe magnesium oxide tunnel barrier layer to form a magnetic tunneljunction, at block 103. FIG. 6C illustrates the amorphous ornano-crystal free layer 18 is then deposited on the magnesium oxidetunnel barrier layer 16 and a cap layer is deposited on the amorphous ornano-crystal free layer 18. The magnetic tunnel junction is thenannealed to crystallize the amorphous ferromagnetic reference and freelayers, as described above, forming coherent interfaces with themagnesium oxide tunnel barrier.

In many embodiments, the amorphous ferromagnetic reference layer 14and/or ferromagnetic free layer 18 can be deposited at a low temperaturesuch as, −50 degrees centigrade or less, to ensure the amorphousstructure formation of the amorphous ferromagnetic reference layer 14and/or ferromagnetic free layer 18.

One particular example of a magnetic tunnel junction includes astructure described in Table 1 below, where the deposition of the layersbegins with layer 1.

TABLE 1 Film stack Layer # Cap Ta or Ru 9B Ru or Ta 9A FLNi_(x)Fe_(y)M_(z) (amorphous magnetic layer) 8B Co_(x3)Fe_(y3)B_(z3) orother nano-crystal magnetic layers 8A MgO MgO or Mg + OX 7 RLCo_(x2)Fe_(y2)B_(z2) or other amorphous magnetic layers 6B Co_(x1)Fe_(y1)B_(z1) or other amorphous magnetic layers 6A Ru Ru 5 PLCo_(x0)Fe_(y0) 4 AFM IrMn 3 Seed Ru 2 Ta 1

Z₃<Z₂<Z₁ for boron content. For instance, Z₃=0-10%, Z₂=6-16%, Z₁=˜20%.X₁>X₂; M can be any additives to enable amorphous NiFe alloy formation.Layer 1˜5 can be conventional layers such as, seed layer,antiferromagnetic layer (AFM), pinned layer (PL), ruthenium interlayerstructure (Ru), respectively. RL refers to the reference layer, and FLrefers to the free layer.

Reference layer 6A, adjacent to Ru can be cobalt rich and has a highboron content to ensure an amorphous layer formation. Also, it may bedesirable to have a high cobalt content to ensure strongRuderman-Kittel-Kasuya-Yoshida (RKKY) synthetic antiferromagnetic (SAF)coupling. Reference layer 6B, adjacent to MgO layer can be iron-richwith low boron content. If grown on an amorphous template and within adesired thickness range, it will grow in an amorphous status due to thenucleation suppression when the boron content is high enough. Due to thelower boron content in 6B, it will start to crystallize at a lowerannealing temperature (e.g., from 250° C. to 300° C. or 300° C. or less)and fully crystallize the reference layer (RL) starting from 6B to 6A.In some embodiments, the layer 6B has a high iron content enablinghigher TMR ratio.

Lowering the deposition temperature of the reference layer (RL) is aneffective way to enable amorphous growth during the deposition of bothlayer 6A and 6B, even with a low boron concentration.

Layer 7 is the tunnel barrier (MgO). Since (001) MgO self-texture isfavored by an amorphous template, the reference layer (6B) should beamorphous to ensure good (001) MgO crystal structure growth. Duringannealing (either at or post deposition), the reference layer (6B and6A) will use such (001) MgO as a template to crystallize and formcoherent interface with the MgO barrier layer.

The free layer 8A is adjacent to the MgO barrier and can be made fromlower boron content CoFeB, either in amorphous or nano-crystal orcrystal structure to form a coherent interface with (001) MgO barrier.The free layer 8B can balance the magnetostriction of the whole freelayer 8. However, it is preferred during/post annealing thatcrystallization does not start from the top cap layer 9/free layer 8Binterface. An amorphous magnetic or non-magnetic layer can be insertedbetween free layer 8A and 8B to suppress the free layer crystallizationfrom locations other than barrier and free layer 8A interface,particularly when 8B is in crystal structure. Thus, an amorphous 8B freelayer is preferred. Another reason why amorphous or nano-crystallinestructure in magnetic layers is preferred in the final product is thatit provides a more uniform rotation of magnetization than apolycrystalline structure. In reader head applications, this can improvethe reader performance by reducing asymmetry sigma. Another reason toemploy an amorphous sub-layer is that it can potentially reducemechanical stress from a top shield, thus reducing noise and improvingreader head stability and symmetry. In this particular example, layer 8Bis made of an NiFeM amorphous layer, it can also be CoFeB layer withhigh enough concentration of boron or other nanocrystal magnetic layersto prevent the crystallization of this layer even after stack anneal.Cap layer 9, can be a single or multiple layer structure, and should notinduce crystallization of the whole free layer from Cap/free layer (FL)interface.

One particular example of a magnetic tunnel junction includes astructure described in Table 2 below, where the deposition of the layersbegins with layer 1.

TABLE 2 Film stack Layer # Cap Ta 9B Ru 9A FL Co-rich amorphousCo_(x4)Fe_(y4)B_(z4) or other layer 8B′ Fe-rich amorphousCo_(x3)Fe_(y3)B_(z3) or other layer 8A′ MgO MgO 7 RL Fe-richCo_(x2)Fe_(y2)B_(z2) or other amorphous layers 6B Co-richCo_(x1)Fe_(y1)B_(z1) or other amorphous layers 6A Ru Ru 5 PLCo_(x0)Fe_(y0) 4 AFM IrMn 3 Seed Ru 2 Ta 1

Layer 9A and 9B are interchangeable and can be either Ta or Ru.X₀˜70˜100%; Y₀=0-30%; X₁>X₂; Y₃>Y₄, Y₂>Y₁; Z₃<Z₂<Z₁ for boron content.For instance, Z₃=0-10%, Z₂=6-16%, Z₁=˜20%, Z₄˜20%.

The above magnetic tunnel junction stacks can be annealed to formcoherent interfaces between the MgO tunnel barrier and the magneticreference layer and free layer. In addition the above magnetic tunneljunction stacks do not have non-magnetic insertion layers such as Ta,Zr, Nb, Ti, and the like, for example, within the either the referencelayer (RL) or the free layer (FL), thus these layers maintain uniformmagnetics. Reducing the boron concentration helps to minimize boronsegregation or migration and reduces magnetic layer non-uniformity. Theabove magnetic tunnel junction stacks also possess improved TMR andmagnetostriction as compared to conventional magnetic tunnel junctionstacks.

A first magnetic tunnel junction (MTJ1) was formed having the followingstructure (thickness value in Angrstroms):Ta(30)/Ru(30)/IrMn(70)/CF₃₀(21)/Ru(8.2)/CoFeB₂₀(12)/CoFeB₁₂(13)/MgO/CoFeB₁₂(20)/Ta(1.5)/NiFe₄(37)/Ru(30)/Ta(60).

A second magnetic tunnel junction (MTJ2) was formed having the followingstructure (thickness value in Angrstroms):Ta(30)/Ru(30)/IrMn(70)/CF₃₀(21)/Ru(8.2)/CoFeB₁₂(25)/MgO/CoFeB₁₂(20)/Ta(1.5)/NiFe₄(37)/Ru(30)/Ta(60).

MTJ1 has a first reference layer=CoFeB₂₀ that is amorphous and serves asa template for amorphous growth of the second reference layer=CoFeB₁₂which, in turn, promotes proper (001) MgO texture, and, as a result, ahigh TMR value (of about 75%) is achieved after the stack is annealed.For MTJ2 the CoFeB₁₂ reference layer does not form an amorphous layerwhen deposited directly on Ru and, as a result, MTJ2 exhibits a low TMR(of about 10%). Even if it does form an amorphous layer as-grown, itssubsequent crystallization during an anneal may start from the bottom Rulayer rather than (001) MgO barrier.

A third magnetic tunnel junction (MTJ3) was formed having the followingstructure (thickness value in Angrstroms):Ta(12)/Ru(20)/IrMn(60)/CF₃₀(20)/Ru(8.2)/(Co₇₅Fe₂₅)B₂₀(10)/(Co₇₃Fe₂₇)B₁₀(13)/MgO/(Co₉₀Fe₁₀)B₄(17)/CoB₂₀(25)/Ta(5)/Ru(30)/Ta(60).This MTJ achieved high TMR without using any non-magnetic Ta insetsanywhere in the stack or NiFe as part of the free layer. The free layeris just 4.2 nm thick with a lowered boron concentration. MTJ3 possessesdesirable soft magnetic properties and negative magnetostriction. Again,low boron concentrations in reference layer 2 and free layer 1 enablecrystallization to start from the MgO template, thus achieving highdegree of coherency and achieving high MR (of about 85%). This stack canbe further modified to further reduce the amount of boron and referencelayer iron for improved SAF coupling and RL/FL homogeneities.

A fourth magnetic tunnel junction (MTJ4) was formed having the followingstructure (thickness value in Angrstroms):Ta(12)/Ru(20)/IrMn(60)/CF₃₀(19)/Ru(8.2)/CoFeB₁₅(10)/—cool down to minus100° C./CoFeB₁₀(12)/MgO/CoFeB₈(12)/CoB₂₀(40)/Ru(30)/Ta60 with a TMR ofabout 35%.

A fifth magnetic tunnel junction (MTJ5) was formed having the followingstructure (thickness value in Angrstroms):Ta(12)/Ru(20)/IrMn(60)/CF₃₀(19)/Ru(8.2)/—cool down to minus 100°C./CoFeB₁₅(10)/CoFeB₁₀(12)/MgO/CoFeB₈(12)/CoB₂₀(40)/Ru(30)/Ta60 with aTMR of about 85%.

These examples demonstrate the benefit of lowering the depositiontemperature to achieve an amorphous CoFeB phase in the reference layernecessary to achieve a high TMR value. The stacks MTJ4 and MTJ5 areidentical; the only difference is the deposition temperature ofreference layer 1=CoFeB₁₅. Low temperature promoted a desired amorphousgrowth of the CoFeB₁₅ film and, as a result, of the reference layer2=CoFeB₁₀ film grown onto the CoFeB₁₅ film; while a room temperatureRT-deposited CoFeB₁₅ layer was partly crystallized, and so is theCoFeB₁₀ deposited onto it, even though it is processed at a lowertemperature (−100° C.) in this example.

Thus, embodiments of the MAGNETIC TUNNEL JUNCTION HAVING COHERENTTUNNELING STRUCTURE are disclosed. The implementations described aboveand other implementations are within the scope of the following claims.One skilled in the art will appreciate that the present disclosure canbe practiced with embodiments other than those disclosed. The disclosedembodiments are presented for purposes of illustration and notlimitation, and the present invention is limited only by the claims thatfollow.

1. A magnetic tunnel junction comprising: an ferromagnetic referencelayer having a first reference layer side and an opposing secondreference layer side, the first reference layer side having a greaterconcentration of boron than the second reference layer side, and thesecond reference side comprising a reference layer crystal structure; amagnesium oxide tunnel barrier layer disposed on the second side of theferromagnetic reference layer, the magnesium oxide tunnel barrier layerhaving a crystal structure; and an amorphous ferromagnetic free layerdisposed on the magnesium oxide tunnel barrier layer wherein theamorphous ferromagnetic reference layer comprises a first sub-layerhaving a first boron concentration and a second sub-layer having asecond boron concentration, and the first boron concentration is greaterthan the second boron concentration.
 2. A magnetic tunnel junctionaccording to claim 1, wherein the first boron concentration is a valuein a range from 10 to 25% atomic and the second boron concentration is avalue in a range from 0 to 15% atomic.
 3. A magnetic tunnel junctionaccording to claim 1, wherein the first sub-layer has a greater cobaltconcentration the second sub-layer.
 4. A magnetic tunnel junctionaccording to claim 1, wherein the second sub-layer has a greater ironconcentration the first sub-layer.
 5. A magnetic tunnel junctionaccording to claim 1, wherein the magnesium oxide tunnel barrier layeris disposed on and in direct contact with the second side of theferromagnetic reference layer.
 6. A magnetic tunnel junction accordingto claim 1, wherein the amorphous ferromagnetic free layer is disposedon and in direct contact with the magnesium oxide tunnel barrier layer.7. A magnetic tunnel junction according to claim 1, wherein the firstsub-layer of the amorphous ferromagnetic free layer is disposed on andin direct contact with the magnesium oxide tunnel barrier layer.
 8. Amagnetic tunnel junction according to claim 1, wherein the firstreference layer side is amorphous.
 9. A magnetic tunnel junctioncomprising: a ferromagnetic reference layer having a first referencelayer side and an opposing second reference layer side, the firstreference side having a greater concentration of boron than the secondreference side, and the second reference side comprising a referencelayer crystal structure; a magnesium oxide tunnel barrier layer disposedon the second side of the ferromagnetic reference layer, the magnesiumoxide tunnel barrier layer having a magnesium oxide crystal structure,wherein the magnesium oxide crystal structure forms a coherent interfacewith the reference layer crystal structure; and a ferromagnetic freelayer disposed on the magnesium oxide tunnel barrier layer, theferromagnetic free layer forming a coherent interface with the magnesiumoxide crystal structure.
 10. A magnetic tunnel junction according toclaim 9, wherein the first reference layer side is amorphous.
 11. Amagnetic tunnel junction according to claim 9, wherein the ferromagneticfree layer has a first free layer side and an opposing second free layerside, the second free layer side having a greater concentration of boronthan the second free side, and the first free layer side being incontact with the magnesium oxide tunnel barrier layer, and the secondfree layer side is amorphous.
 12. A magnetic tunnel junction accordingto claim 9, wherein the ferromagnetic reference layer has a boronconcentration gradient extending between the first reference layer sideand the opposing second reference layer side.
 13. A magnetic tunneljunction according to claim 11, wherein the ferromagnetic free layer hasa boron concentration gradient extending between the first free layerside and the opposing second free layer side.
 14. A magnetic tunneljunction according to claim 9, wherein the ferromagnetic reference layercomprises a first sub-layer having a first boron concentration and asecond sub-layer having a second boron concentration, and the firstboron concentration is greater than the second boron concentration. 15.A magnetic tunnel junction according to claim 14, wherein the firstboron concentration is a value in a range from 10 to 25% atomic and thesecond boron concentration is a value in a range from 0 to 15% atomic.16. A method comprising: depositing an amorphous ferromagnetic referencelayer having a first reference layer side and an opposing secondreference layer side, the first reference side having a greaterconcentration of boron than the second reference side; depositing amagnesium oxide tunnel barrier layer disposed on the second side of theamorphous ferromagnetic reference layer, the magnesium oxide tunnelbarrier layer having a crystal structure; and depositing an amorphous ornano-crystal ferromagnetic free layer on the magnesium oxide tunnelbarrier layer, forming a magnetic tunnel junction.
 17. A methodaccording to claim 16, further comprising annealing the magnetic tunneljunction and initiating crystallization of the amorphous ferromagneticreference layer at the second reference layer side and initiatingcrystallization of the amorphous or nano-crystal ferromagnetic freelayer at an interface between the amorphous ferromagnetic free layer andthe magnesium oxide tunnel barrier layer.
 18. A method according toclaim 17, wherein the crystallization initiates at 300 degreescentigrade or less.
 19. A method according to claim 16, wherein thedepositing an amorphous ferromagnetic reference layer or amorphousferromagnetic free layer occurs at a temperature of −50 degreescentigrade or less.
 20. A method according to claim 16, furthercomprising depositing a cap layer on the amorphous ferromagnetic freelayer, the cap layer preventing or suppressing crystallizationinitiation at an interface between the cap layer and the amorphousferromagnetic free layer.